1. Field of the Invention
This invention relates to a heat treatment apparatus for forming a film on a treatment object such as a semiconductor wafer.
2. Description of the Related Art
In general, in a process for manufacturing a semiconductor integrated circuit, a metal, such as W (tungsten), WSi (tungsten silicide), Ti (titanium), TiN (titanium nitride), TiSi (titanium silicide), an alloy thereof, or a compound thereof is deposited on a wafer, thereby forming a thin film thereon.
As an apparatus for forming such a thin film, a heat treatment apparatus 2 with heating lamps, as shown in FIG. 11, is used. The heat treatment apparatus 2 has a cylindrical treatment container 4 made of aluminum, etc., and a thin mount table 8 contained in the container 4 and made of e.g. carbon material or an aluminum compound. The mount table 8 is supported, separated from the bottom wall of the container 4, by a support member 7 which has a cylindrical base and three support arms upwardly extending at regular intervals from the upper surface of the base. A transparent window 10 made of quartz is provided below the table 8 such that it closes an opening formed in the bottom wall of the container 4. A sub-box is attached to the bottom wall of the container 4 around the opening, in which heating means 12 or lamps are provided to be rotatable in a direction indicated by the arrow.
At the time of forming a film, the interior of the treatment container 4 is kept airtight, and the heating means 12 is lighted. Heat rays from the heating means 12 are transmitted to the mount table 8 through the transparent window 10, whereby a semiconductor wafer W mounted on the mount table 8 is indirectly heated to and kept at a predetermined temperature. During heating, a process gas, e.g. WF.sub.6, and a reduction gas, e.g. H.sub.2, are uniformly applied above the surface of the wafer from a shower head 14 provided above the mount table 8, thereby forming on the wafer a metallic thin film of tungsten, etc. To prevent the process gas from flowing into the space defined below the mount table 8, i.e. within the support member 7, and forming an opaque film on the under side of the table 8 and on the transparent window 10, there is provided a gas introducing pipe 11 for introducing, into the space, a backside gas consisting of an inactive gas such as Ar gas or N.sub.2 gas. Thus, undesirable flowing of the process gas to an outer peripheral portion or an under surface portion of the wafer is avoided. (See Jpn. Pat. Appln. KOKAI Publications Nos. 4-124820 and 6-120145)
When in a semiconductor integrated circuit having, for example, an MOSFET structure, an electrode or a wire made of aluminum or tungsten is formed directly on an electrode-forming portion of an Si substrate by CVD, it is possible that the material of the electrode or wire will absorb Si from the substrate to make Si deposit on the electrode, or that tungsten atoms will enter the Si wafer, thereby increasing the contact resistance of the electrode-forming portion. To restrain the absorption of Si or the entrance of tungsten, a barrier metal film, which shows high stability to heat, etc., is interposed between the electrode and the electrode-forming portion of the substrate. Depending upon the kinds of the materials of the electrode and the electrode-forming portion, the material of the barrier metal film is selected from high-fusion metals such as Ti, W and Mo, silicide of the metals, and TiN. Further, the barrier metal film is formed by sputtering.
In the case shown in FIG. 12, a TiN barrier metal film 16 is formed on the silicon wafer W, and a tungsten film 18 serving as a wire or electrode is formed on the barrier metal film 16. The barrier metal film 16 prevents separation of Si into the electrode.
As aforementioned, the barrier metal film or TiN film 16 is generally formed by sputtering, while the tungsten film 18 is generally formed by CVD using the heat treatment apparatus described referring to FIG. 11.
Since in the sputtering method, the direction in which a film is formed is highly controlled, the edge of the TiN thin film 16 formed by sputtering can be accurately aligned with the edge of the wafer W. On the other hand, in the CVD method, any step will inevitably be covered. Therefore, the process gas will reach even an under surface portion of the wafer W, thereby forming the tungsten film 18 which extends even to the under surface portion of the wafer W. Thus, an unnecessary film portion or portions 20 are formed as shown in FIG. 12.
The unnecessary film portion 20 is directly attached to the Si wafer W, with no barrier metal film interposed therebetween. Further, the thickness of the film portion 20 is ununiform and thin. Accordingly, it may easily peel off the wafer. Peeled metal may well raise particles.
Since the wafer W is mounted tightly on the mount table 8, it does not seem that the process gas will undesirably reach the under surface of the wafer. Actually, however, a slight clearance is defined therebetween. In the case of the tungsten film formed by CVD, it can sufficiently cover even a fine hole with a diameter of about 0.5 micrometer. This being so, the process gas can easily reach the under surface of the wafer through the slight clearance. If, to prevent it, the flow of the backside gas supplied into the space below the mount table 8 is increased, the backside gas may well enter the reaction chamber above the table 8 to thereby reduce the concentration of the process gas. As a result, the film formed on the wafer will be partially thin, i.e. the inplane uniformity of the resultant film thickness will be degraded. In light of this, the flow of the backside gas cannot be increased.
U.S. Pat. No. 5,304,248 discloses another type film-forming apparatus. In this apparatus, a vertically movable shield ring is provided on the outer periphery of a wafer mount table. When a susceptor is moved to a position in which film-forming is performed, the overall upper surface of a peripheral portion of a wafer placed on the susceptor is clamped by the inner lip of the shield ring. Also in this apparatus, a sufficient sealing effect cannot be obtained, and it is highly possible that the process gas will leak to the under surface of the wafer, thereby forming a film thereon. After careful examination of the cause of the leakage, the following was found: Since a peripheral portion of the wafer is brought into surface contact with the inner lip of the shield ring, the airtightness at the contacting interface between the wafer and the shield ring is microscopically too low to sufficiently prevent leakage of the process gas through the interface. As a result, an unnecessary film will inevitably be formed.